The recent discovery of superconductivity in a (La, Ba) cuprate led to worldwide research activity which very quickly resulted in the discovery of other metal oxides having relatively high superconductive transition temperature T.sub.c. In particular, it was discovered that YBa.sub.2 Cu.sub.3 O.sub.7 can have T.sub.c of about 90 K. [See, for instance, R. J. Cava et al, Physical Review Letters, 58, 1676 (1987).]
To date, the research efforts have resulted in the identification of two classes of high T.sub.c oxide superconductors. The first class has nominal composition La.sub.2-x M.sub.x CuO.sub.4-.epsilon., where M denotes one or more divalent metals such as Ba, Sr, or Ca. The members of this first class have been found to have transition temperatures ranging up to about 40 K.
The second class of metal oxide superconductors has nominal composition Ba.sub.2-y (M.sub.1-x M'.sub.x).sub.1+y Cu.sub.3 O.sub.9-.delta., where M and M' are chosen from Y, Eu, Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb, Lu, La, Sc, Sr or combinations thereof. Typically, 0.ltoreq..times..ltoreq.1, 0.ltoreq.y.ltoreq.1, and 1&lt;.delta.&lt;3. See, for instance, D. W. Murphy et al, Physical Review Letters, 58(18), 1888 (1987). Herein, the former class of compounds will be referred to as the La-cuprate system, and the latter as the Ba-cuprate system. Many members of the Ba-cuprate system have T.sub.c greater than 77 K., the boiling point of liquid nitrogen.
Exemplary of the Ba-cuprate system are YBa.sub.2 Cu.sub.3 O.sub.7, EuBa.sub.2 Cu.sub.3 O.sub.7, and La.sub.1.5 Ba.sub.1.5 Cu.sub.3 O.sub.7. (It will be understood that chemical formulae of oxide superconductors herein are approximate only, and that deviations may occur. For instance, the optimal oxygen content in YBa.sub.2 Cu.sub.3 O.sub.7 frequently is not 7 but about 6.9).
Recently, there have been reports that incorporation of fluorine into at least one composition in the Ba-cuprate system can yield a superconductive material with relatively high transition temperature. [S. R. Ovshinsky et al, Physical Review Letter, 58 2579 (1987).]
A multitude of applications for the novel high T.sub.c oxide superconductors have been proposed, and many of the proposed applications require the formation of a thin film (e.g., less than about 5 .mu.m thickness) of the superconductive material on a substrate. Exemplary of such applications are superconductive interconnects between electronic devices, chips, or assemblies, and superconductive components such as Josephson junctions and SQUIDs.
Superconductive oxide films have been produced by a variety of techniques, including evaporation or sputtering in a reactive (e.g., oxygen-containing) atmosphere, sputtering from oxide targets, and spin-on of a metal-containing solution. See, for instance, R .H. Koch et al, Extended Abstracts-High Temperature Superconductors, Proceedings of Symposium S, 1987 Spring Meeting of the Materials Research Society, Apr. 23, 1987, Anaheim, Calif. (Materials Research Society, Pittsburg, PA, 1987), page 81; R. H. Hammond et al, Proceedings of Symposium S, supra, page 169; P. Chaudhari et al, Physical Review Letters, 58, 2684 (1987), (evaporation of yttrium, barium, and copper in an oxygen atmosphere, followed by annealing of the resulting oxygen-containing film at temperatures in the range of 850.degree. C. to 910.degree. C.). See also R. E. Somekh et al, Nature, 326, page 857 (1987), (sputtering from Cu, BaO and Y.sub.2 O.sub.3 sources onto a sapphire substrate held at 1050.degree. C. followed by an anneal in oxygen at 500.degree. C.). M. Moriwaki et al, in Proceedings of Symposium S, supra, page 85, described production of films by sputtering, but details of the process were not given. The spin-on technique is disclosed in U.S. patent application Ser. No. 037,264, "Method for Producing a Superconductive Oxide Layer, and Apparatus Comprising Such a Layer", filed Apr. 10, 1987 for C. E. Rice. A further deposition technique (using multiple sources including a source comprising a fluoride of a Group IIA element) is disclosed in U.S. patent application Ser. No. 089,296, "Robust Superconductors", filed Aug. 25, 1987 for R. E. Howard et al.
Although all of the above techniques have been successfully employed to produce superconductive oxide films, the prior art techniques typically do have certain shortcomings. For instance, the reactive sputtering technique is relatively slow, and typically poses difficult control problems due to the uncertainties associated with the density of as-deposited material, which typically has unknown oxygen content. Sputtering from an oxide target is also difficult to control due, inter alia, to the fact that the target surface typically becomes progressively depleted in, e.g., Ba. On the other hand, it is frequently difficult to produce oxide films of a thickness in excess of about 1 .mu.m by the spin-on technique. Furthermore, at least some prior art techniques for patterning superconductive oxide films frequently result in a deterioration of the superconductive properties of the films.
In view of the potential technological importance of superconductive oxide films, a method for forming such films that is easily controllable and relatively rapid and which can be used to form relatively smooth films, including relatively thick films, would be highly desirable. This application discloses such a method. Disclosed is also a method for patterning metal oxide films that typically does not result in deterioration of the superconductive properties of the films.